This invention relates generally to the processing of a substrate utilizing a plasma in the production of integrated circuits, and specifically relates to the determination of substrate bias parameters in a plasma processing system, such as one utilizing an electrostatic chuck to secure a substrate to a susceptor during processing.
Gas plasmas are widely used in a variety of integrated circuit fabrication processes, including plasma etching and plasma deposition applications, such as PECVD. Generally, plasmas are produced within a processing chamber by introducing a low-pressure process gas into the chamber and then directing electrical energy into the chamber for creating an electric field therein. The electric field creates an electron flow within the chamber which ionizes individual gas molecules by transferring kinetic energy to the molecules through individual electron-gas molecule collisions. The electrons are accelerated within the electric field, producing efficient ionization of the gas molecules. The ionized particles of the gas and the free electrons collectively form what is referred to as a gas plasma.
Gas plasmas are useful in a variety of different processes. One commonly used plasma process is a plasma etch process wherein a layer of material is removed or xe2x80x9cetchedxe2x80x9d from a surface of a substrate. In an etch process, the ionized gas particles of the plasma are generally positively charged, and the substrate is negatively biased such that the positively ionized plasma particles are attracted to the substrate surface to bombard the surface and thereby etch the substrate surface. For example, a substrate might be etched to remove an undesirable material layer or coating on the substrate before another layer is deposited. Such a pre-deposition etch process is often referred to as etch cleaning of the substrate.
Other common plasma processes involve deposition, wherein a material layer is deposited upon the substrate. Chemical vapor deposition, or CVD, for example, generally involves the introduction of material gases into a processing chamber wherein the gases chemically interact and form a material layer or coating on the exposed substrate surface. A gas plasma can be utilized to enhance the chemical interaction. Consequently, such a CVD deposition process utilizing a plasma is referred to as plasma-enhanced CVD or PECVD. The plasma is utilized to provide energy to the process and enhance the deposition quality and/or deposition rate. Other plasma deposition processes also exist as are commonly understood by a person of ordinary skill in the art.
During plasma processing of a semiconductor substrate, it is often useful to apply an accelerating voltage to the surface of the substrate. The accelerating voltage or substrate bias is utilized to accelerate ions or other charged particles within the plasma to the substrate surface. In an etch process, the charged plasma particles are attracted to the substrate surface to actually bombard the surface and provide the etch as discussed above. In a deposition process, such as PECVD, the energy provided by such charged particle bombardment may be utilized to further enhance the deposition rate or to enhance the deposition quality, as mentioned above.
Generally, biasing of the substrate in plasma enhanced etch and deposition processes is accomplished by capacitatively coupling an RF field from electrodes in the processing chamber, through the substrate, and to the exposed substrate surface which is to be etched, or which is to receive a deposited material layer. Specifically, the electrodes, which are positioned within a susceptor or substrate support, are biased with an RF power supply to create an RF field. The RF field is then capacitatively coupled through the susceptor and substrate to create a relatively uniform DC bias potential across the exposed substrate surface. The substrate surface DC bias, in turn, affects the plasma, as discussed above, to enhance the etch or deposition process.
Within a plasma processing system, the plasma will usually have particular non-uniformities associated therewith. For example, the plasma density is often greatest in the center of the plasma, due to edge effects proximate the sides of the processing chamber. The non-uniformities in the plasma translate to discrepancies within the etch and deposition processes in which the plasma is utilized. For example, an undesirable variation in etch rate may occur wherein the etch rate proximate the center of the substrate is greater than the etch rate proximate the outer edges of the substrate. Furthermore, within a plasma-enhanced deposition process, the deposition may be affected proximate the center of the substrate differently than at the edge of the substrate thus creating a non-uniform deposition layer and a non-uniform deposition rate radially across the substrate.
Attempts have been made in the art to address such plasma non-uniformities in a plasma processing system. For example, U.S. Patent Application entitled xe2x80x9cImproved Apparatus and Method for Plasma Processing of a Substrate Utilizing an Electrostatic Chuck,xe2x80x9d filed May 4, 2000, discloses a plasma processing system which selectively adjusts the bias on the substrate to offset plasma non-uniformities in the system; that application is incorporated herein by reference in its entirety. While that system improves the overall plasma process, it has been difficult to achieve precise selectivity in varying the substrate bias. Therefore, it is an objective of the present invention to provide more precise adjustments to the substrate bias in a plasma processing system for addressing non-uniformities in the plasma.
In accordance with another aspect of the invention, it is desirable to provide precise bias control even in a system utilizing an electrostatic chuck. Particularly during integrated circuit fabrication, the substrate being processed is supported within the processing chamber by a substrate support or susceptor. Oftentimes, the substrate is physically secured on the susceptor during processing, such as to improve heat transfer between the substrate and susceptor. One way of securing a substrate involves the use of an electrostatic chuck (ESC), which uses an applied DC bias to the substrate to electrostatically attract and secure the substrate to the susceptor. Electrostatic chucks are known in the art with suitable designs being shown in U. S. Patent Application entitled xe2x80x9cImproved Apparatus and Method for Plasma Processing of a Substrate Utilizing an Electrostatic Chuck,xe2x80x9d filed May 4, 2000, noted above, and in U.S. Pat. No. 5,117,121, which is also incorporated herein by reference. Electrostatic chucks will usually use the same electrodes as are used to bias the substrate. This practice has made precise measurement of the substrate surface bias levels even more difficult due to the effect of the electrostatic clamping voltage on such measurement. Therefore, it is a further objective to provide more precise biasing of a substrate to address plasma nonuniformities within a processing system utilizing an electrostatic chuck.
It is still another objective of this invention to address the above-discussed objectives without adversely affecting the desired biasing of the substrate surface which is necessary for plasma processing.
The processing system, in accordance with the principles of the present invention, provides more precise monitoring of substrate bias within a plasma processing system for improved substrate bias control. Additionally, the inventive system provides such precise bias measurements in combination with a system utilizing an electrostatic chuck. Measurements provided by the system may be used to vary the bias across selected portions of the exposed substrate surface for more uniform plasma processing, or selectively varied plasma processing.
To that end, the processing system comprises a processing chamber for containing a plasma. A substrate support is mounted within the chamber for supporting the substrate proximate the plasma. A plurality of electrodes, such as first and second electrodes, are coupled to the substrate support and are positioned proximate the supporting surface. The electrodes are electrically isolated from one another. An RF power source is coupled to each of the electrodes for biasing the electrodes with RF electrical energy. The biased electrodes are operable in conjunction with the substrate support for creating a DC bias on the exposed surface of the substrate positioned on the supporting surface of the substrate support.
In accordance with one aspect of the present invention, a plurality of comparators are utilized to monitor the bias on the individual electrodes for the purpose of selectively varying the bias. More specifically, the first comparator has first and second inputs which are electrically coupled to one of the electrodes. An isolating device is coupled between the inputs and is operable for isolating the first input from the bias on the monitored electrode which is created by the RF power source. The comparator has an output reflective of a voltage difference between the first and second inputs which results from the electrode bias created by the RF power source. Since the first input is isolated from the electrode bias created by the RF power source, the output of the first comparator is reflective of electrode RF bias.
A second comparator provides an indication of the voltage-bias differential between the first and second electrodes. With the output from the first comparator coupled to the one electrode, and the output of the second comparator coupled between both electrodes, the bias level on each of the individual electrodes may be obtained. Specifically, the second comparator has a first input coupled to the first electrode and a second input coupled to the second electrode. The comparator has output reflective of a voltage difference between the first and second inputs which results from the bias difference between the first and second electrodes. With the signal reflective of the bias on one electrode and a signal reflective of the bias differential between the two electrodes, the relative bias on the other, unmeasured electrode, may be obtained. In that way, the relative bias on the first and second electrodes may be monitored so that the bias may be optimally adjusted.
The monitored outputs of the comparators may be utilized to determine the relative bias on the electrodes so that the bias may be adjusted as desired for the plasma process. To that end, variable capacitors are coupled between the RF power source and the electrodes. The capacitance of the capacitors is selectively varied for varying the bias created on the substrate by at least one of the electrodes relative to the bias created on the substrate by the other of the electrodes. In that way, the effect of the plasma on one portion of the substrate relative to the effect of the plasma on another portion of the substrate may be selectively varied.
In accordance with another aspect of the present invention, at least one of the inputs to the comparators is coupled to the respective electrodes through a voltage dividing circuit. The voltage dividing circuit includes a variable resistor which may be selectively adjusted to vary the voltage level at the comparator input. When the processing system utilizes an electrostatic chuck, a DC power source is coupled to the electrodes in addition to the RF power source. The DC power source provides a voltage differential at the electrodes which is necessary for electrostatically clamping the substrate to a susceptor. The variable resistors may be selectively varied to eliminate the effect of the clamping DC bias on the electrodes at the inputs to the comparators. In that way, the outputs of the comparators are directly related to the differences between the inputs caused by the DC bias on the electrodes created by the RF power source.
In accordance with another aspect of the present invention, another comparator is utilized having first and second inputs coupled to the first and second electrodes, respectively. The output of the comparator is also reflective of the voltage difference between the first and second inputs resulting from the bias difference between the first and second electrodes. However, the output is coupled to an automatic adjustment device, such as a servo motor, which is, in turn, coupled to the variable resistor. The servo motor automatically varies the resistance of the variable resistor in order to adjust one of the inputs and to zero the output from the comparator. The output of the comparator drives the servo and the variable resistor until the output is zero. When this is done with the DC power source on and the RF power source off, the bias measurement is essentially not affected by the DC power source because the inputs to the comparator coupled to both the electrodes are adjusted to generate a zero output. Since the inputs to the comparator providing automatic adjustment of the variable resistor are the same as the inputs to the comparator which measures the different bias between the electrodes, automatically zeroing the output of the one comparator will zero the output of the other comparator as well. A disabling switch is coupled between the output of the respective comparator and the automatic adjustment device. The disabling switch is coupled to the RF power source and is operable for disabling the automatic adjustment device when the RF power is delivered to the electrodes. In that, the output of the comparator coupled to both electrodes will not be further varied to zero its output, and therefore it will measure the voltage differential between its first and second inputs to provide an output signal reflective of the bias differential between the electrodes.
Therefore, the signals are generated at the output of the comparators to be reflective of the individual bias levels of the electrodes which are attributable to the RF power supply. In that way, the variable capacitors may be adjusted to achieve the desired electrode bias between the two electrodes. Accordingly, the biasing of the substrate may be similarly selectively varied to achieve the desired results within the plasma process. While the present invention is described with respect to two electrodes, multiple electrodes may also be utilized and their bias levels measured accordingly, utilizing the present invention.
These features and benefits of the invention, and other features and benefits are set forth in greater detail in the Detailed Description hereinbelow made in reference to the drawing figures.